Sand molds for metal casting and methods therefor



United States Patent O 3,212,144 SAND MOLDS FOR METAL CASTING AND METHODS THEREFOR Frank R. Capps, PD. Box 3335, Jacksonville, Fla. No Drawing. Filed June 22, 1962, Ser. No. 204,600 11 Claims. (Cl. 22-129) This invention relates to molding materials employed in the preparation of sand molds for casting metals, and more particularly, to essentially moisture-free molding materials which can be formed into molds for casting metals, recovered after the casting step, and subsequently reused in the preparation of additional molds. The invention also relates to rigid self-supporting sand molds produced from the molding materials, to methods for producing the sand molds, and to methods for subsequently casting metals therewith.

Materials employed in preparing molds for casting metals may be classified generally as reusable and nonreusable. Non-reusable molding materials commonly employed include plasters, concrete, and ceramics. In preparing molds having the desired hardness from nonreusable materials the character of the molding mixture is altered to such a degree that it is impossible or impractical to attempt to recover and reuse any of the ingredients thereof. Although in most cases castings of excellent quality can be obtained from molds prepared from non-reusable materials, the cost of supplying fresh molding material for each individual casting usually renders the utilization thereof impractical in commercial casting operations.

The reusable molding materials most commonly employed in metal casting processes usually consist of mixtures of silica sand, clay, and Water in combination with an additional material which serves as a binder in the final mold. The amount of water employed in these materials generally is of a magnitude of about 4% by Weight of the total mass. In the casting of metals with molds prepared from such moisture-containing materials, a substantial portion of the moisture in the mold is converted to steam. The porosity characteristics of these molds must be such to permit any steam generated to escape from the interior thereof in a direction away from the casting surface. Otherwise, steam produced within the molds undesirably will cause the surface of the casting to be pitted and may even blow molten metal out of the mold cavity. Consequently, the operational suitability of such moisture-containing molds is partly contingent upon the porosity thereof being satisfactorily controlled.

On the other hand, molds employed for casting metals must be sufficiently dense and have sufficient strength to maintain their shape during the casting step. When the conventional moisture-containing molding materials are compressed under pressures which provide increasingly dense molds, the molds become correspondingly less porous and finally are rendered unsuitable for the reasons stated hereinabove. As a result, commercial metal casting processes employing such molding materials limit the pressures used in compacting the materials to values well below about 150 p.s.i. with about 75 p.s.i. being that which is the standard.

Mixtures of sand, clay, and Water themselves do not develop molds having the requisite hardnesses and strengths for casting purposes when compacted under such pressures, and the ordinary procedure is to incorporate thereinto an additional material as a supplemental binder, which imparts the necessary rigidity to the mold structure during a heating or baking operation carried out preliminary to the casting step. Substances utilized as binders in these molding materials include coal, pitch, molasses, cereals, resins and the like. The use of such 3,212,144 Patented Oct. 19, 1965 supplemental binders, however, is not completely satisfactory. The heating or baking step required therewith sacrifices valuable production time, and the sand recovered after the casting step usually is so contaminated with residue from the binder that appreciable proportions of it are unsuitable for reuse. Furthermore, when casting metals with molds containing such binders, particularly casting at high temperatures, a considerable amount of the molding material is burned and charred at the mold surface. Hence, in processes utilizing such molding materials, significant proportions of the molding material, including sand and binder, undesirably must be separated and discarded from the system.

In such heating and baking steps carried out in the preparation of the molds, the mold materials are pressed in a confined space against a pattern which has a raised surface corresponding to the cavity desired in the final mold, heat is applied thereto until the desired hardness is achieved, and then the mold and pattern are separated. The molds formed from these conventional molding materials, however, are not easily separable from the pattern surfaces, and generally at additional expense it is necessary to employ a coating of a parting compound upon the pattern and/or mold surface. Even when parting compounds are employed, the formed molds usually must be vibrated or jarred loose from the surface of the pattern. Such separation procedures effect a slight distortion in the configuration of the mold cavity, and consequently accurate and uniform castings are not obtained.

In addition, heating or baking the molding material while pressed in relationship with the pattern surface may cause the pattern to become slightly warped with repeated usage. Therefore, when a relatively high degree of accuracy is required in the casting operation, the patterns employed in forming the molds must be continuously replaced.

Molds formed from moisture-containing molding materials conventionally employed also are characterized by further disadvantages. The surfaces of castings made in such molds, even though relatively smooth, undesirably may not be adapted for machine finishing. This is a result of the casting surface being chilled by steam generated in the mold by the molten metal charge. In some instances, especially in the case of the high temperature metals, it may be necessary to anneal or stress relieve the resulting casting. In addition, due to their reactivity with water some metals such as magnesium cannot be satisfactorily cast in such molds.

To alleviate some of the problems occasioned by the presence of moisture in metal casting molds, reusable molding materials have been suggested which contain liquid substitutes for water in the binder. The substitutes most commonly employed are relatively high boiling substances derived from petroleum. While these substances overcome the disadvantages accruing by virtue of the volatility of water, the utilization thereof usually does not provide molds which are completely satisfactory since in many cases the molds do not display the requisite hardness, and at the temperatures attendant casting operations the hydrocarbonaceous materials generally will smoke profusely and may even flame up and burn. Moreover, molding materials containing such binders usually have a characteristic stickiness which undesirably makes them difiicult to handle and convey through processing equipment.

With regard to casting metals, such as magnesium, which are normally reactive with water, it has been suggested to substitute polyhydric alcohols for Water in the casting molds, as is exemplified by U.S. Patent No. 1,- 533,892. The molds described in U.S. 1,533,892, however, are not completely satisfactory since the molds described therein are not characterized by sufficient standup compression strength to provide highly accurate castings unless employed inconjunction with an external supporting means such as a foundry flask and the like.

It will be understood that as employed hereinafter in the specification and claims, the term self-supporting refers to molds which are characterized by suflicient compression strengths to be non-deformable when subjected to pressures normally encountered in metal casting methods, and consequently can be utilized in accurately casting metals without the need of external supporting means such as foundry flasks and the like.

Accordingly, it is the primary object of the present invention to provide a sand mold which is more advantageously adapted for casting metals than any mold heretofore available and formed from a molding material so characterized that substantially all of it can be recovered after the casting step and repeatedly reused in the preparation of additional molds.

It is another object of the present invention to provide a sand mold which can be employed more economically in metal casting processes than molds heretofore available.

It is an additional object of the present invention to provide a sand mold which is suitably adapted to be employed universially in casting either low or high temperature metals.

It is a further object of the present invention to provide a sand mold which suffers only relatively small losses of the components therein by vaporization or burning when subjected to temperatures normally encountered in metal casting processes.

It is yet another object of the present invention to provide a sand mold which is inert with respect to metals cast therein.

Still another object of the present invention is to provide a sand mold which provides accurate, clean metal castings having smooth surfaces.

Yet a further object of the present invention is to provide a sand mold for casting metals which can be separated cleanly and easily from pattern surfaces employed in the formation of the desired mold cavity without the necessity of using parting compounds or jarring the mold.

Still an additional object of the present invention is to provide a sand mold which suitably can be employed to provide castings of high temperature metals having surfaces which are satisfactory for machine finishing.

It is a particular object of the present invention to provide a rigid sand mold formed from reusable materials in which metals advantageously can be cast and which is characterized by sufficient strength to be self-supporting and resist deformation during the casting step.

It is yet another particular embodiment of the present invention to provide a method for forming sand molds from reusable materials wherein without the use of heat or hardening agents, rigid, self-supporting molds are produced which advantageously can be employed in casting metals.

It is still another particular object of the present invention to provide a continuous method for casting metals wherein a rigid self-supporting mold initially is prepared from dry, non-sticy molding materials, a metal casting of excellent characteristics is formed in the self-supporting molds, and after the casting is removed from the mold substantially all of the components thereof are recovered and employed in preparing additional molds.

Broadly described, the present invention provides a rigid, self-supporting sand mold for casting metals consisting of an essentially moisture-free molding mixture compressed to at least about 300 p.s.i., said molding mixture consisting essentially of sand, clay, and a polyol with the sand preferably being smooth-grained.

In addition the present invention also provides a method for producing a sand mold from resuable materials which comprises admixing sand, preferably smooth-grained sand, clay, and a polyol to provide an essentially moisturefree mixture, compressing said mixture against a pattern in a confined chamber to a pressure of at least about 300 p.s.i., and separating said compressed mixture from said pattern and chamber in the form of a rigid, self-supporting mold having a cavity therein corresponding to the configuration of said pattern.

Another feature of the present invention is to provide a method for casting metals in said rigid, self-supporting molds.

The sand molds of the present invention are rigid and self-supporting, being characterized by relatively high densities and compression strengths as compared to sand molds normally employed. Consequently, metal casting advantageously can be carried out in the molds of the present invention without the necessity of the molds being externally supported. The molds are formed from dry, non-sticky mold materials in a simple compression step thereby eliminating the use of a heating or baking step in the mold-forming operation. The highly compressed molds separate freely from the surface of the pattern employed to provide the desired cavity configuration therein Without the need for parting compounds to be employed. Separation of the molds from the pattern surface is accomplished without vibration or jarring and as a result molds formed accordng to the present invention are characterized by a relatively high degree of accuracy and uniformity. Moreover, due to the elimination of the heating or baking step in the mold-formation process by employing the method of the present invention, the effective life of mold-forming patterns is substantially increased.

Since the molds of the present invention are essentially moisture-free and contain relatively high boiling liquids in the binders, substantially reduced amounts of gaseous substances are generated during the casting steps. Thus, by the use of molds formed according to the present invention, castings :may be produced which are characterized by the desired smooth surfaces. Moreover, when using the molds of the present invention, there is no danger that the molten metal will be blown out of the mold cavity or pitted by excessive vaporization of constituents in the mold. The limited vaporization of components in the molds of the instant invention further allows castings to be made without the surfaces of the metal being overly chilled with the result that any desired machine finishing thereof can be carried out easily. Moreover, the gaseous substances formed during the casting step are not obnoxious.

Even at high casting temperatures only a thin crust of scorched material is formed on the surfaces of the present molds. Hence, in accordance with the method of the present invention it is possible to maintain the losses of molding material at values which are substantially lower than those observed generally in conventional casting operations.

Although the molds of the present invention are characterized by having relatively high densities and compression strengths, the molds need only be subjected to a slight impact in order to be disintegrated. In such a disintegrated state the material easily can be remulled in conventional foundry equipment to be placed in condition for reutilization.

The sand constituent contemplated to be utilized in preparing the molds of the present invention suitably may be any foundry-type sand but preferably smoothgrained sand is to'be used as substantially improved results are obtained thereby. As used hereinafter in the specification and claims, the term foundry-sand is meant to refer to a sand or other refractory material having a particle size distribution range which is typical of granular materials conventionally employed in foundry practice. The term smooth-grained as employed in the specification and claims to describe the sands preferred for use in the present invention is intended to embrace sands having grains with rounded surfaces or which are substantially spherical in configuration. Under the recognized classification of sand grain shapes em ployed in foundry practice the preferred sands for use in connection with the present invention are designated as having rounded sand grains, although sub-angular grain sands which have somewhat less smooth surfaces can be employed with somewhat reduced strength in final mold and even angular grain sands are acceptable if the other conditions of the invention are adhered to and the lesser mold strength obtained is satisfactory for the items being cast therein.

The actual size distribution is not critical although the distribution should be relatively uniform throughout the particular particle size range. The particle size distribution range of a typical foundry-type sand is as follows.

Mesh: Percent -40 4 max 50 10-15 ZOO-pan 6 max.

The actual type of sand preferred for utilization in preparing a specific mold depends primarily upon the particular use to which the mold is to be put. For example, the sand which is preferred for utilization in preparing the molds of the invention employed in casting high melting temperature metals such as iron and steel is zirconite sand. Zirconite sand is characterized by a relatively high specific gravity, fusing point and heat conductivity as compared to ordinary silica sand and is found in nature having smooth grains. The heat conducting properties of zirconite sand allow the high temperature metals to be casted in molds prepared therefrom with only insignificant losses of the molding material employed. Heat from the high temperature casting is dissipated throughout the mold and thus high temperature penetration of the mold surfaces and accompanying burning out are greatly reduced. By virtue of its above-mentioned properties, zirconite sand also is well suited for use in preparing mold-s of the invention intended for utilization in casting the ligher and lower melting temperature metals. When it is desired to cast metals in relatively thin sections, it is desirable to have a minimum of heat loss from the molten metal as it flows into the mold cavity. For such instances, molds formed from silica sand or other lighter refractory materials are particularly adapted if the molds are characterized by the requisite strength. In accordance with the present invention, such sands having smooth round grains are employed and the resultant molds display the desired strength characteristics.

The clay constituent of the molding materials contemplated for use in preparing the sand molds of the present invention may be any clay conventionally employed in preparing metal casting molds. It will be understood that as employed hereinafter in the specification and claims the term clay is meant to refer to such alumino silicates generally. The bentonites constitute the preferred embodiments of the clay constituent with western bentonite being that clay which is more particularly preferred for utilization.

Polyols which suitably can be employed in the preparation of the sand molds of the present invention include dihydric alcohols such as glycols, diglycols, and polyglycols and polyhydric alcohols such as glycerol, erythritol, arabitol, mannitol and sorbitol. The alkylene glycols provide the preferred materials for use in forming the molds of the present invention. Propylene glycol is the more particularly preferred binder agent due to its cheapness and availability.

In preparing the sand molds of the present invention, the sand, clay and polyol constituents suitably may be admixed in any expedient means by adding the constituents in any desired order. Since sand and clay both 6 normally contain a small amount of moisture, it may be necessary to subject the clay and sand to a drying step preliminary to the combining of the ingredients comprising the molding material. Usually a moisture content in the molding materials on the order of about 0.5% by weight is tolerable. Any suitable drying method can be employed in accordance with the method of the present invention. Polyols are hygroscopic and absorb moisture from the atmosphere. Thus, if the polyol employed contains a significant amount of moisture, it also is desirable to subject the polyol to a drying step preliminary to the admixture of the molding material components. If small amounts of water are present in the polyol due to the relatively small proportion of polyol in the final molding material, it usually is not necessary to effect the removal thereof. In some cases, for example, when zirconite sand, western bentonite and propylene glycol are employed in forming the molds, a small amount of water present in the fresh molding material will slightly improve some properties of the molding material as a reusable material.

The clay and polyol serve as a binder for sand particles in forming the desired casting molds. The relative proportion of binder to sand should be such that each particle of the smooth-grained sand is covered only with a thin coating of binder so that when the molding material is ultimately compressed, the ordinarily occurring voids in the material will not be completely filled with binder and render the sand mass mass impermeable. Of course, the actual amounts of sand, clay and polyol employed in any specific instance Will depend upon the characteristics of the particular sand, clay and polyol utilized. As a general rule, sand should constitute about by weight of the molding material with the remainder being composed of a clay-polyol binder in Which the weight ratio of clay to polyol is about 4: 1. The amount of clay employed usually is in the range of from about 3 to about 18, preferably from about 4 to about 15, parts by weight per parts sand. The amount of polyol present in the binder usually is such that the weight ratio of clay to polyol is in the range of from about 2:1 to about 6:1, preferably from about 3 :1 to about 5 1. In the preferred embodiment of the present invention wherein zirconite sand, western bentonite and propylene glycol are employed, the amount of bentonite preferred in the molding mixture is in the range of from about 3 to about 12, more preferably from about 4 to about 10, parts by weight per 100 parts of the sand with the amount of glycol present being such as to provide the above-stated claypolyol weight ratios.

The mixture of ingredients comprising the molding mixture is then placed in a confined chamber which suitably may be of any configuration, although it is preferred that it be cylindrical. Due to the fact that the molding materials used in the invention are dry, nonsticky and flow like ordinary sand, the material easily can be placed in the chamber such as by pouring, blowing and the like. The mixture in the chamber is then compressed against a conventional pattern plate providing one end of the chamber to a pressure of at least about 300 p.s.i., preferably at least about 500 p.s.i., without any heat being applied thereto. Although pressures greater than about 1000 psi. suitably can be employed in accordance with the method of the present invention, the advantages gained by the utilization of such pressures are more than offset by the additional expenditures attendant thereto. Accordingly, the economy of the molding operation generally dictates the use of pressures below 1000 p.s.i. in the mold-forming step. The molding material is maintained under pressure for a time period requisite for the binder to set and provide a strong, rigid sand mass. In accordance with the present method, even though no heat or hardening agents are employed, the sand mass sets to the desired hardness in seconds. Of course, the time actually required for a specific molding material to harden satisfactorily will depend upon the particular composition thereof. When zirconite sand, western bentonite and propylene glycol are employed in forming the molds of the present invention, the time period usually employed is at least about 2 seconds and preferably about 3 or 4 seconds. Although periods longer than about 5 seconds suitably may be employed, the use thereof does not provide any additional advantage and only unnecessarily slows down mold production.

By means of the relatively short mold-forming time achieved by the invention, the overall casting operation advantageously may be greately accelerated. In addition, the total reusable molding material requirements of a system producing a given amount of castings per unit time correspondingly may be proportionately reduced.

After the molding materials have hardened to the desired degree, the resulting compressed mixture is then separated from said pattern and chamber in the form of a rigid and self-suporting mold provided therein with a cavity corresponding to the configuration of the pattern.

The molds thus provided are easily separated from the surface of the pattern. It is not necessary to employ parting compounds or to vibrate or jar the compressed mold in order to effect the separation of the mold and pattern surfaces. Accordingly, after the separation, the surface of the pattern is clean and the cavity of the mold is an accurate representation of the pattern. The resulting molds are rigid and self-supporting, being characterized by sufiicient compressive strength to withstand relatively high compressive forces without deformation or fracture.

A molten metal is then poured into the cavity of the mold and allowed to cool in order to provide the desired casting. Although an external supporting means such as a conventional foundry flask may be employed in the casting step, due to the relatively high density and relatively high strength of the molds of the present invention, the utilization thereof is not necessary, and preferably the casting step is carried out with the molds in an unsupported state. Metals which suitably can be cast by the use of the molds of the present invention include both high and low temperatures metals such as iron,

steel, magnesium, aluminum, brass and the like.

When the molten metal is poured into the mold cavity, the hot metal may cause a relatively small amount of gaseous substances to be generated within the mold. Although characterized by a relatively high density, the molds of the present invention also are sufficiently porous to allow vapor produced in the casting step to escape through the mold away from the casting surface.

After the casting in the mold has hardened and cooled to a sufiicient degree, the casting easily is removed and separated from the mold. The castings thereby obtained are accurate reproductions of the patterns utilized in forming the molds and have relatively clean surfaces. Any sand clinging thereto easily may be removed by a slight brushing. Due to the limited vaporization of the binder in the molds of the invention, the castings obtained therefrom are smooth and unpitted. In addition, chilling of the surface of the casting is maintained at a minimum during the casting step which enables any desired machine finishing of the resulting castings to be carried out with relative ease.

The used molds may be, and preferably are, then subjected to a treatment to recover the components thereof for reutilization in the casting operation. When the materials comprising the molds are desired to be recovered and reused, the mold initially is disintegrated. Although the molds of the present invention are characterized by relatively high compressive strengths, they may readily be disintegrated by merely subjecting them to a slight impact as is accomplished by dropping them a short distance. The disintegrated molding material is then placed in condition for reuse in the system by adding amounts of fresh ingredients to compensate for losses in the casting step and subsequently reworking the resulting mixture. The reworking of the material suitably may be accomplished by mulling or otherwise subjecting the sand to the pulverized treatment as by the use of a ball mill which fractures the bonds between the individual sand grains without fracturing the grains themselves.

Any loss of the molding material incurred in the casting step is made up by the addition of appropriate amounts of sand, clay and/ or polyol. In accordance with the present invention, the amounts of materials which are required to replace losses are relatively small. Even when high temperature metals are cast in the molds of the present invention, the total amount of material required to compensate for losses is only on the order of about 24% by weight. The material loss essentially is polyol and, consequently, the expenditure of refurbishing the molding material is insubstantial.

A thin crust of scale formed from said, clay and burned polyol is produced on the surface of the mold in the casting step. The smaller flakes of this scale are beneficial to the sand. In order not to overload the system with the scale, however, it usually is desirable to effect a separation of at least a portion thereof preliminary to the molding material being reused in the system. The desired degree of separation of the scale suitably may be accomplished by any means such as by dropping the mold into a conventional foundry shakeout screen which upon the disintegration of the mold selectively screens out the larger flakes of scale from the molding sand.

Although coarse separated scale may be discarded, preferably this material is accumulated, the carbon and binder residue scrubbed off the sand therein, and the sand returned to the system.

Although the above description of the sands employed in preparing the molds of the present invention emphasizes the importance of and improved results obtained by use of those sands which have smooth rounded grains, it will be apparent to those skilled in the art that the present invention contemplates molds formed from molding materials wherein rough-grained sand also is present in minor amounts which do not destroy the characteristics imparted to the molds by the smooth-grained sand constituent. This is particularly the case when the rough-grained sand employed is of an average particle size which corresponds to the smaller grains of the smooth-grained sand.

Since other modifications of the compositions, articles and methods of the invent-ion which do not depart from the scope of the description will become apparent from the general description and specific embodiments appearing in the specification, it is intended that this invent-ion be limited solely by the scope of the appended claims.

What I claim is:

1. A rigid, self-supporting sand mold for casting metals consisting of an essentially moisture-free molding mixture compressed to at least about 300 psi, said molding mixture consisting essentially of zirconite sand, bentonite in an amount in the range of from about 3 to about 12 parts by weight per parts said sand, and polyol, the weight ratio of said bentonite to said polyol being in the range of from about 2:1 to about 6:1.

2. The sand mold according to claim 1 wherein said mixture is compressed to a pressure in the range of from about 500 to about 1000 psi.

3. A method for producing a sand mold from reusable materials which comprises admixing essentially moisturefree smooth-grained sand, bentonite and a polyol to provide an essentially moisture-free mixture, compressing said mixture against a pattern in a confined chamber at a pressure of at least about 300 p.s.-i., and separating said compressed mixture from said pattern and chamber in the form of a rigid, self-supporting mold having a cavity therein corresponding to the configuration of said pattern.

4. The method according to claim 3 wherein said mixture is compressed to a pressure in the range of from about 500 to 1000 psi.

5. The method according to claim 3 wherein said sand is Zirconite sand.

6. The method according to claim 3 wherein said polyol is an alkylene glycol.

7. A method for producing a sand mold from reusable materials which comprises admixing essentially moisturefree zirconite sand, bentonite, and a polyol to provide an essentially moisture-free mixture, said mixture containing from about 3 to about 12 parts bentonite per 100 parts by weight of said sand and having a weight ratio of said bentonite to said polyol in the range of from about 2:1 to about 6:1, com-pressing said mixture against a pattern in a confined chamber at a pressure of at least about 300 p.=s.i., and separating said compressed mixture from said pattern and chamber in the form of a rigid, self-supporting mold having a cavity therein corresponding to the configuration of said pattern.

8. The method according to claim 7 wherein said mixture is compressed at pres-sure in the range of from about 500 to 1000 p;s.i.

9. A molding material consisting essentially of an essentially moisture-free mixture of zirconite sand, bentonite and polyol, said mixture containing from about 3 to about 15 parts by weight 'bentonite per 100 parts said sand and having a weight ratio of said ben-tonite to said polyol in the range of about 2: 1 to 6: 1.

10. The molding material according to claim 9 wherein said clay is Western bentonite.

11. The molding material according to claim 9 wherein said polyol is propylene glycol.

References Cited by the Examiner UNITED STATES PATENTS 1,533,892 4/25 Osborne 222l6.5

1,868,456 7/32 Wood 22216.5

2,871,527 2/52 Wallwork 222O OTHER REFERENCES Barlow: High-Pressure Molding, reprinted from Foundry, March 1956.

MARCUS U. LYONS, Primary Examiner.

MICHAEL V. BRINDISI, Examiner. 

1. A RIGID, SELF-SUPPORTING SAND MOLD FOR CASTING METALS CONSISTING OF AN ESSENTIALLY MOISTURE-FREE MOLDING MIXTURE COMPRESSED TO AT LEAST ABOUT 300 P.S.I., SAID MOLDING MIXTURE CONSISTING ESSENTIALLY OF ZIRCONITE SAND, BENTONITE IN AN AMOUNT IN THE RANGE OF FROM ABOUT 3 TO ABOUT 12 PARTS BY WEIGHT PER 100 PARTS SAID SAND, AND POLYOL, THE WEIGHT RATIO OF SAID BENTONITE TO SAID POLYOL BEING IN THE RANGE OF FROM ABOUT 2:1 TO ABOUT 6:1. 